This invention relates generally to the measurement of electrostatic potentials, and more particularly to an apparatus for achieving the modulation of a sensor or electrode used to measure an electrostatic field or electrostatic voltage.
The electrostatic voltmeter which includes the present invention is a device capable of measuring electrostatic fields or potential without current flow through the device. Generally, these devices include a probe or sensor assembly working in conjunction with an associated voltmeter assembly which receives the signals from the probe and produces an output signal. Subsequently, the output signal may be used to drive an indicator, or to control an electrostatic process as a function of the measured electrostatic potential. Thus, the features of the present invention may be used in the printing arts and, more particularly, in an electroreprographic system to control a xerographic process. These electrostatic voltmeters are particularly well suited for measuring photoreceptor surface charge, which in turn allows for the automated adjustment of machine characteristics to achieve high quality reprographic output.
Heretofore, it has been established that a sensing electrode must be modulated with respect to the field being measured in order to accurately measure the field. Essentially, two methods of achieving the required modulation of the electrode are known. The first method requires that the electrode be stationary and that a vibrating element, or vane, be moved between a viewing port and the electrode itself to modulate the field which reaches the electrode. The second method utilizes a moving electrode which is vibrated relative to the surface being measured. Two embodiments of this method are commonly used. A first embodiment moves the electrode in a direction perpendicular to the surface being measured thereby directly varying the capacitance between the electrode and the surface. The second embodiment utilizes an aperture, placed between the electrode and the surface under test, to periodically capacitively couple the electrode and the surface through the aperture as the electrode is moved parallel to the surface. This may be done by affixing the electrode to the end of a vibrating device such as the tine of a tuning fork. An advantage of the second embodiment is that if the electrode is generally centered on the aperture, the motion of the electrode will decouple it from the surface twice during every vibration cycle, effectively doubling the frequency of the modulation as compared to the frequency of the vibrating system. Generally, both the first and the second embodiments utilize a tuning fork to reliably achieve the modulation of the electrode, either by a vane attached to one end of the fork, or by affixing the electrode directly to an end of the fork.
The approaches devised to vibrate the electrode in an electrostatic field in order to measure the electrostatic surface potential, are illustrated in the following disclosures which may be relevant:
U.S. Pat. No. 4,763,078, patentee: Williams, issued: Aug. 9, 1988;
U.S. Pat. No. 4,720,682, patentee: Ikushima et al. issued: Jan. 19, 1988;
U.S. Pat. No. 4,625,176, patentee: Champion et al. issued: Nov. 25, 1986;
U.S. Pat. No. 4,614,908, patentee: Daniele et al. issued: Sep. 30, 1986;
U.S. Pat. No. 4,318,042, patentee: Eda et al.; issued: Mar. 2, 1982;
U.S. Pat. No. 4,149,119, patentee: Buchheit, issued: Apr. 10, 1979;
U.S. Pat. No. 3,921,087, patentee: Vosteen, issued: Nov. 18, 1975;
U.S. Pat. No. 3,852,667, patentee: Williams et al., issued: Dec. 3, 1974.
The relevant portions of the foregoing patents may be briefly summarized as follows:
U.S. Pat. No. 4,763,078 to Williams relates to a sensor for an electrostatic voltmeter which consists of a vibratory element supported on one end in the manner of a cantilever beam, a sensitive electrode on the vibratory element for measuring the potential, a driver for vibrating the vibratory element in a direction to vary the capacitive coupling between the electrode and the electrical field being measured, and an amplifier mounted directly on the vibratory element so as to be in synchronous motion with the electrode. When the vibrating element is at rest, it is disposed in a plane substantially parallel to the plane of a base plate.
U.S. Pat. No. 4,720,682 to Ikushima et al. discloses a surface electric potential sensor for detecting the potential on a surface in a non-contacting fashion. The sensor consists of a tuning fork, a piezoelectric vibration element mounted on each leg of the tuning fork, at a position near a base of the tuning fork, for vibrating the tuning fork, an insulating substrate mounted on a free end of each leg of the tuning fork, a conductive film provided on the insulating substrate, a board for supporting the tuning fork, and a shield case for encasing the tuning fork.
U.S. Pat. No. 4,625,176 to Champion et al. describes a vibrating probe for measuring electrostatic potential associated with electrophotographic copiers and print machines. The probe is made from a single piezoceramic bender element surrounded by a shield at a known potential. The bender element is comprised of a piezoceramic drive layer connected to a drive oscillator, a sensing conductor, and a piezoceramic feedback layer which is also connected to the drive oscillator. This system provides a simple electrostatic probe comprised of a single bender element which acts as a drive element, a sensor element, and a feedback element in one.
U.S. Pat. No. 4,614,908 to Daniele et al. relates to a probe for electrostatic voltmeters which measures the voltage on a photoconductive surface. The probe consists of a microdeflector which includes a base having a well and a flexible finger on the base, positioned over the well. The finger is able to deflect relative to the base. The finger further includes an inner electrode and an outer electrode.
U.S. Pat. No. 4,318,042 to Eda et al. relates to an electrometer probe for measuring the electrostatic potential on the surface of a photoconductive drum, in an electrostatic machine. The probe includes an electrode which is in the form of a strip.
U.S. Pat. No. 4,149,119 to Buchheit teaches an electrostatic voltmeter or electrometer which includes a probe sensor for sensing electrostatic charge present on a test surface. The probe sensor is modulated using a rotating vane or shutter arrangement. The probe is also conditioned to receive both A.C. and D.C. signals which are amplified by a D.C. amplifier, where the A.C. signal from the probe is fed back to the D.C. amplifier to stabilize its output.
U.S. Pat. No. 3,921,087 to Vosteen discloses a capacitive electrostatic modulator for an electrostatic voltmeter. The modulator includes tines, or vanes, operatively associated with each of the ends of the tuning fork and placed between a sensitive electrode and the source of the electrostatic field being measured. A driver is used to vibrate the tuning fork, and thus the vanes, thereby alternatively coupling and decoupling the electrode with the electrostatic field, and producing a voltage corresponding to the magnitude of the field. Associated circuitry is used to connect the electrostatic modulator to a voltmeter or field meter.
U.S. Pat. No. 3,852,667 to Williams et al. relates to a probe or sensor for an electrostatic voltmeter including a voltage sensitive electrode which is vibrated within a housing so as to vary the amount of the surface of the electrode which is directly exposed to an external electrical potential through an aperture in the housing. The electrode is connected to one tine of a tuning fork which is vibrated when a driving signal is applied to a piezoelectric transducer on the fork. An electrical conductor, which connects the electrode to the voltmeter circuitry, is provided with an electrically driven shield, and a reference potential is applied to the housing to enhance the accuracy of the signal generated by the electrode.
An object of the present invention is to provide a single balanced beam electrode modulator capable of modulating the field to which an electrostatic electrode is coupled. The balanced beam structure results in a simple configuration which enables a compact design and the physical separation of the vibratory drive signal and the sensed signal produced by the electrode. Likewise, the error in the sensor signal, caused by coupling between the power signal supplied to the vibratory driver and the electrode signal, may also be reduced.
In accordance with the present invention, there is provided an electrostatic measurement apparatus having a single balanced beam vibratory element which is resiliently supported. The apparatus also includes a driver for vibrating the single balanced beam element. The single balanced beam element has an electrode, operatively associated therewith, suitable for sensing a capacitive coupling relationship with an electrostatic field, whereby the vibration of the single balanced beam vibratory element results in the modulation of the capacitive coupling relationship between the electrode and the electrostatic field.